This application relates to an improved cooling process for thermoplastic film webs as well as to closure profiles secured to film webs for use on reclosable plastic bags or containers, and to the use of surfactants to aid in the cooling process.
For a considerable period of time, plastic bags or containers which feature reclosable fasteners have been widely used by consumers and industry. Typically, the interlocking closure profiles on the container are pressed together or pulled apart to seal or open the container. Although many different processes have been developed to manufacture such closure profiles, most processes in the art utilize an extrusion die in which a closure profile of a thermoplastic resin such as polyethylene is extruded and subsequently joined to a thermoplastic film web or sheet. A common method involves joining the extruded closure profiles onto the film while hot so that a firm bond is formed. Consequently, a cooling step is generally included in the manufacturing process to facilitate removal of heat from the freshly extruded profiles to cool and stabilize the profile shapes. Water is a well known coolant used in the cooling step.
For example, in U.S. Pat. No. 4,540,537, Kamp discusses the use of an extrusion die for producing closure profiles. After extrusion, the closure profiles enter a cooling bath containing water.
The process taught by Wagers in U.S. Pat. No. 4,764,977 involves extruding the closure profiles and subsequently securing them to an underlying plastic film web where water is then sprayed onto the extruded profiles on the film web to cool them.
Scheibner et al, U.S. Pat. No. 4,676,851 also teach extruding interlocking closure profiles, joining them to a film surface and subsequently directing a flow of liquid coolant such liquid nitrogen, chilled brine, or water onto the closure profiles through cooling nozzles.
Noguchi, U.S. Pat. No. 3,945,872 also relates to the extrusion of closure profiles which are bonded to a film surface, and teaches the use of a jet mechanism containing a gas or liquid coolant such as water for cooling the extruded profiles.
Sutrina et al, U.S. Pat. No. 4,295,919, also relates to a process for fusing closure profiles to separately extruded film and then utilizing water spray jets to cool the closure profiles.
Although water is widely used as a cooling agent, water has a high surface tension which prevents it from effectively wetting the surface of thermoplastic resins such as polyethylene, which has a much lower surface tension. The high surface tension of water causes it to "bead up" on the film, resulting in a slow transfer of heat from the closure profiles to the water. In many extrusion apparatuses, air jets have been used in conjunction with water to aid in the cooling process. However, this method becomes inefficient as manufacturing speeds increase because the time allotted for cooling the profiles after bonding to the underlying film web decreases. Insufficient cooling of the profiles may lead to deformation of the still warm and soft profiles at subsequent stages of the manufacturing process. This, in turn, may result in unacceptable zipper performance on finished bags.
Accordingly, a need exists in the art for an improved method for cooling freshly extruded film webs, particularly interlocking closure profiles on those webs.